Add SVGRenderBase to share logic between SVG renderers
        https://bugs.webkit.org/show_bug.cgi?id=25532

        I've added an SVGRenderBase base class which is shared by
        all of the SVG renders now.  This patch is just moving code
        there are no functional changes in this patch.

        Currently I've left these functions in SVGRenderSupport.h
        In a later patch I will rename SVGRenderSupport.* to SVGRenderBase.*

        clampImageBufferSizeToViewport now takes a FrameView* which makes some code easier to read.

        * rendering/RenderSVGBlock.h:
        * rendering/RenderSVGImage.h:
        * rendering/RenderSVGModelObject.h:
        * rendering/RenderSVGRoot.h:
        * rendering/SVGRenderSupport.cpp:
        (WebCore::SVGRenderBase::prepareToRenderSVGContent):
        (WebCore::SVGRenderBase::finishRenderSVGContent):
        (WebCore::clampImageBufferSizeToViewport):
        (WebCore::SVGRenderBase::computeContainerBoundingBox):
        (WebCore::SVGRenderBase::filterBoundingBoxForRenderer):
        * rendering/SVGRenderSupport.h:
        * rendering/SVGRootInlineBox.cpp:
        (WebCore::SVGRootInlineBoxPaintWalker::chunkStartCallback):
        (WebCore::SVGRootInlineBoxPaintWalker::chunkEndCallback):
        (WebCore::SVGRootInlineBox::paint):
        * svg/SVGMaskElement.cpp:
        (WebCore::SVGMaskElement::drawMaskerContent):
        * svg/SVGPatternElement.cpp:
        (WebCore::SVGPatternElement::buildPattern):
        * svg/graphics/SVGPaintServerGradient.cpp:
        (WebCore::createMaskAndSwapContextForTextGradient):
        (WebCore::clipToTextMask):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@43209 268f45cc-cd09-0410-ab3c-d52691b4dbfc